A shown in FIG. 3, a conventional refrigerant compressor comprises a closed vessel 1 housing an electric motor 4 comprised of a stator 2 and a rotor 3, and a compression equipment 5 disposed under the electric motor 4. The compression equipment 5 has a suction pipe 6, to which an accumulator 7 is attached. A delivery pipe 8 extends from a top portion of the closed vessel 1.
The refrigerant compressor of this arrangement is generally incorporated into air conditioners, refrigerators and the like. In keeping with the current trend towards energy savings, there is a strong demand for an improved efficiency of the compression mechanism as well as an improved efficiency of the electric motor.
The improvement of efficiency of the electric motor is primarily accomplished by increasing core lengths of the stator and the rotor or by changing the coil winding specifications (or increasing the number of turns) for the stator and the rotor. In both cases, however, an increased earth capacitance of the electric motor results in an increased current leakage from the air conditioner or the refrigerator.
That is, because the stator 2 and rotor 3 constituting the electric motor 4 for the refrigerant compressor are housed inside a closed vessel 1 together with the compression equipment 5, they are exposed to a refrigerant gas having a higher dielectric constant than that of the air, so that the earth capacitance of the electric motor increases.
Because of this, there has been a problem that current leakage is liable to occur in the electric motor for the refrigerant compressor.
It has been found that such current leakage can be effectively reduced by decreasing the earth capacitance of the refrigerant compressor, i.e., by reducing the electrostatic capacity of the electric motor housed inside the closed vessel.
However, such countermeasures as to decrease the thickness of a laminated core of a stator and a rotator or to decrease the number of turns of wiring will lower the efficiency of the electric motor. Thus, none of the countermeasures can be effective.
In view of this, the earth capacitance of the electric motor is reduced by enhancing the insulation characteristic of a slot. The enhanced insulation characteristic of the slot may be accomplished by decreasing the surface area of the winding inside the slot or of the slot itself, or otherwise by applying an insulation film of a greater thickness or of a multi-layered structure to the slot.
In the case of enhancing insulation characteristic by means of slot insulation film, polyethylene terephthalate (hereinafter referred to as "PET") is generally used for the insulation film for use in the electric motor 4 because of the general characteristics thereof including insulating performance, durability, mechanical strength, costs and the like. In cases where the efficiency of the motor is to be increased by increasing the number of turns of the stator, an insulation film of a great thickness or of double-layered structure is applied to the slot for the reduction of the earth capacitance.
However, the volume of the wound coil insertable in the slot and the thickness of the insulation film are in a reciprocal relation to each other. Therefore, the efficiency of the electric motor has been increased to such a level that the further effect can not be provided to reduce the earth capacitance of the refrigerant compressor.
On the other hand, in case where polyethylene naphthalate (hereinafter referred to as "PEN") having a low dielectric constant, for example, is used for the slot insulation film for use in the refrigerant compressor, the resultant film is stable in a refrigerant and a lubricant, and is also effective at reducing the earth capacitance. Unfortunately, however, the film has poor mechanical characteristics, or a poor tear strength, in particular. Accordingly, when the insulation film is inserted in the narrow slot prior to the coil, a tear may be produced in the film at an end portion thereof. The coil subsequently fitted in the slot is caused to expose itself from such tear, leading to an insulation failure taking place between the coil and the stator and/or rotor core.